1. Field of the Invention
The present invention relates to a backlight unit, and more particularly to a backlight unit that is capable of making it possible to highly condense light and reduces costs at the same time.
2. Description of the Background Art
Generally, a liquid crystal display LCD controls the transmittance of a light beam supplied from a backlight unit to display a desired picture on a screen by a liquid crystal display panel consisting of a plurality of liquid crystal cells arranged in matrix type form, and a plurality of control switches for switching the video signals to be supplied to each liquid crystal cell.
In conjunction with FIG. 1, there is explained a backlight unit as follows.
Referring to FIG. 1, a conventional backlight unit includes a light guide panel 4 for guiding a light beam via a light incident part 20; a reflecting plate 2 located under the light guide panel 4 for reflecting a light beam, which progresses to a lower surface and side surfaces of the light guide panel 4, to the upper surface; the first diffusion sheet 6 for diffusing the light beam passed through the light guide panel 4; the first and second prism sheet 8 and 10 for controlling the progress direction of the light beam passed through the first diffusion sheet 6; and the second diffusion sheet 12 for diffusing the light via the prism sheets 8 and 10 and protecting the prism sheets. The light incident part 20 consists of a lamp 22 for generating a light beam and a lamp housing 24 having the lamp 22 mounted on and reflecting the light beam to the light guide panel 4.
A cold cathode fluorescent lamp is mainly used as the lamp 22, and the light generated from the lamp 22 is incident on the light guide panel 4 through the incident surface located at the side surface of the light guide panel 4. The lamp housing 24 has a reflecting surface inside to reflect the light from the lamp 22 to the incident surface of the light guide panel 4. The light guide panel 4 makes it possible to deliver the light incident from the lamp 22 to where it is far away from the lamp 22. The reflecting plate 2 is installed at the lower surface of the light guide panel 4 to face it.
The reflecting plate 2 reflects again the light incident on itself through the rear surface of the light guide panel 4 to the light guide panel 4, thereby reducing light loss. A printed pattern formed at the lower surface of the light guide panel 4 is provide so that the light beam passed through the light incident part is reflected in a specific angle at the rear surface, being an inclined surface, to uniformly progress to an out-coming surface. At this moment, the light progressing to the lower surface and the side surface of the light guide panel 4 is reflected at the reflecting plate 2 to progress to the out-coming surface. The light come out via the light guide panel 4 diffuses light to the whole area.
On the other hand, the light incident on a liquid crystal display panel (not shown) has the highest light efficiency when perpendicular to the liquid crystal display panel. Therefore, it is desirable to deposit two forward directional prism sheets in order to make the progress angle of the light, which has come out of the light guide panel 4, perpendicular to the liquid crystal display panel. The light passed through the first and the second prism sheet 8 and 10 is incident on the liquid crystal display panel via the second diffusion sheet 12. The conventional backlight unit with such a composition can has a desired viewing angle profile only when it has two prism sheets. In this case, there are problems in that light loss is increased and manufacturing costs are increased as well.
There is illustrated in FIG. 2, a structure proposed to solve the problem. In conjunction with FIG. 2, it is explained as follows.
Referring to FIG. 2, a conventional backlight unit includes a light guide panel 4 for guiding a light beam via the light incident part 20, a reflecting plate 2 located at the lower part of the light guide panel 4 for reflecting a light beam progressing toward the lower and side surface of the light guide panel 4 to an upper surface thereof, a reverse directional prism sheet 14 controlling the progress direction of the light beam passed through the light guide panel 4, and a diffusion sheet 12 for diffusing a light beam passed through the reverse directional prism sheet 14.
The function and operation of the light incident part 20 and the reflecting plate 2 is explained sufficiently enough in FIG. 1 so that the detailed description will be omitted here.
A relief prism pattern provided at the lower surface of the light guide panel 4 is insufficient to satisfy a total reflection condition of the light beam passed through the light incident part 20 so that the light beam is made to progress to the upper surface uniformly. A reverse directional prism sheet 14 is arranged at the upper surface of the light guide panel 4. In this case, because the angle of the light beam coming out of the light guide panel 4 is over approximately 65°, it is desirable to keep the vertical angle of the prism sheet 14 between 63° and 70°.
Accordingly, the light beam passed through the prism sheet 14 becomes perpendicular to the liquid crystal display panel. The light beam passed through the prism sheet 14 is diffused and is progressed to the entire area by the diffusion sheet 12. The conventional backlight unit with such a composition has a problem in that the wall surface of the light guide panel 4 is mirrored or the bright line of the light incident part 20 is visible due to the use of the reverse directional prism sheet 14.
Therefore, there is an urgent need for a backlight unit that is capable of highly condensing light and minimizing the wall surface of the light guide panel 4 being mirrored and visibility of the bright line of the light incident part 20.
Such a backlight unit condenses light by use of a condensing sheet 16 with good condensing efficiency as shown in FIGS. 3 and 4.
The backlight unit for highly condensing light, shown in FIG. 3, has a relief pattern provided at the lower surface of the light guide panel 4 to provide a function of the prism sheets 10 and 8 and the diffusion sheets 6 and 12, as shown in FIG. 1. The light beam via the light guide panel 4 is passed through the condensing sheet 16 to improve its light condensing efficiency.
The backlight unit for highly condensing light, shown in FIG. 4, has a relief pattern formed at the lower surface of the light guide panel 4 and a prism foil 18 formed at the light out-coming area of the light incident part 20 for condensing the light that comes out of the lamp 22.
However, because the condensing sheet 16 shown in FIGS. 3 and 4 is so expensive that it is only used for aeronautical or military purposes. It can hardly be used for general purposes. Also, there is a problem in that the backlight unit for highly condensing light, shown in FIG. 4, has a size as large as the area where the prism foil 18 is formed.